Why do some assembly references have a version and others not when added through NuGet

Why do some assembly references have a version and others not when added
through NuGet

I'm adding all packages through NuGet, so I'm not manually tweaking the
Specific Version property in Visual Studio. However depending on which
package (or even which version) I add, I end up with different ways of how
the reference is added. As you can see below:
A reference to Autofac is added without a version.
A reference to AutoMapper is added, but the assembly version is added as
well. Version=3.0.0.0, Culture=neutral, processorArchitecture=MSIL"
Even stranger is the fact that the stable NuGet package of AutoMapper
doesn't add the version either (make sure you save the csproj file between
changing packages). What's the reason that packages added through the same
method (NuGet install package) results in different configurations?
<Reference Include="Autofac">
<HintPath>..\packages\Autofac.3.1.1\lib\portable-win+net40+sl50+wp8\Autofac.dll</HintPath>
</Reference>
<Reference Include="AutoMapper, Version=3.0.0.0, Culture=neutral,
processorArchitecture=MSIL">
<SpecificVersion>False</SpecificVersion>
<HintPath>..\packages\AutoMapper.3.0.0-ci1043\lib\windows8\AutoMapper.dll</HintPath>
</Reference>